Elastomers have been widely used as springs, both in tension and in compression. One of the more recent elastomers is a copolyester polymer. They can broadly be described as randomly joined soft and hard segments. They are well known and are described, among other places, in U.S. Pat. Nos. 3,763,109, 3,766,146 and 3,651,014. One such polymer, sold under the trademark HYTREL is made from three ingredients, namely, (1) dimethyl terephthalate, (2) polyglycols such as polytetramethylene ether glycol, polyethylene ether glycol or polypropylene ether glycol, and (3) short chain diols like butanediol and ethylene glycol. When these are reacted to form high molecular weight polymers the diol/terephthalate blocks form crystalline hard segments, and the softer amorphous phase contains units of ether glycol terephthalate. In actual practice I have used HYTREL 5550, 5555, 5556, 5556HS, 4056 and 6356. The first two numbers of these compositions signify the durometer hardness on the D scale.
Such copolyester polymers act suitably when employed as tension springs. However, they have proven to be quite unsuitable for use as compression springs. This is due to the fact that when compressed over about ten percent they commence taking on a permanent set. Obviously, a spring that permanently deforms under load is quite unsuitable for the intended purpose. The applications in which the spring will not be compressed over about ten percent are quite limited (i.e., little spring action is required).
The principal object of the present invention is to provide a method by which such copolyester polymer elastomers may be processed to produce a product which will serve as a compression spring in applications wherein the force applied to the spring is such as to compress the spring significantly over ten percent. Alternatively stated, the object is to produce a copolyester polymer compression spring which will not take a significant permanent set when operating under conditions such that the spring is compressed substantially over ten percent. In accordance with the present invention, this is achieved by annealing a block of the copolyester polymer elastomer and then precompressing that block by applying a force to the block sufficient to compress the block an extent greater than thirty percent of its previous dimension measured in the direction of the applied pressure.
One use for elastomeric springs has been in railroad draft gears. In such applications it is known to use metal plates bonded to each side of the elastomeric pad and thereby obtain substantially improved work capacity from the pad. See for example U.S. Pat. Nos. 2,713,483 and 3,227,288. With the elastomeric materials commonly employed in the past a good bond between the pad and a metal plate can be obtained with commercially available adhesives. However, it has not been possible to find an adhesive that will adequately bond to the copolyester polymers referred to above.
In one embodiment, a copolyester polymer compression cushioning unit is constructed in accordance with the invention by placing a pair of multi-apertured plates on opposite sides of an initially precompressed block and applying a second precompressive force to the block and plates sufficient to cause the copolyester polymer to flow into the apertures in the plates and form a mechanical bond between the block and the plates. The plates may be formed with projections adjacent the apertures to enhance the mechanical bond between the plates and the block when the spring unit is assembled. To additionally increase the force capacity of such an elastomeric spring, the faces of the plates bearing against the pad are roughened, as by means of sandblasting.
Other features of the invention will be ascertained from the following description.